Should I use Unity ECS or Entitas for a rts simulation shared between client and server..

Hello Unitians,

I'm wondering what is the best approach to develop a rts simulation that needs to run on client & server side..

So my question is..
Should I develop the RTS simulation as a pure c sharp dll, and share it with unity and the backend?
This approach will make me use Entitas framework.

Or should I use Unity ECS then create a server build? I'm lost here..
Imagine I want to develop a Clash Royale game. Clash Royale game has 'the 'Home" screen where you collect boxes and then the "Battle"(the simulation)..

So what would be your approach?
Thanks.